when to replace 02 sensor

[lytnin88]

I have a 2003 Forester with 67K miles on it. Owner's manual doesn't say anything about when to replace 02 sensor. Should I just wait until I get CEL?

Also, when should I change spark plug wires? Still have factory wires. I checked resistance recently when I changed spark plugs, and all wires are between 6.1K and 7.9K ohms.

[Legacy777]

O2 sensors are typically ok until about 100k. They may get lazy with less mileage, but as good preventative maintenance, 100k is a good number.

[OB99W]

Owner's manual doesn't say anything about when to replace 02 sensor. Should I just wait until I get CEL?[...]Also, when should I change spark plug wires?

I tend to keep tabs on performance/mpg, and (besides the CEL) a change in either would motivate me to consider those things. So, absent bad mpg, misfiring, etc., I wouldn't worry about them at your current mileage. Now if a mouse has nibbled off the plug wire insulation, that's another thing .

 

As to the resistance measurements, they seem in the ballpark. It's likely that anything from about 5-10 kohms wouldn't be problematical. Ignition voltage is high, but the current is not; a small increase in wire resistance won't make much of a difference at the plug. BTW, in addition to providing EMI (interference) suppression, the resistance also slightly "slows" the spark. That actually can be beneficial, giving a bit longer time for ignition; people who install low-resistance ignition wires thinking that performance will be improved may find the opposite, especially if the plugs are also non-resistance type.

[color-blind]

In theory you should never need to replace the sensor until the check engine light is on. Yet that is not allways the case. Many Subaru vehicles will have their air fuel ratio sensor fail and not store a code. To catch this you need to monitor fuel trims and see if they are out of spec. The only way you would know is by a drop in gas mileage, hesitiation, lack of power or intermittent stalling. Unless you have a driveability issue I would leave it alone untill the check engine light comes on. I had one here today with 150,000 miles on the original unit. Hope this helps.

[idosubaru]

(interference) suppression, the resistance also slightly "slows" the spark.

 

at 186,282 mph i don't know if those electrons are being slowed enough to be significant over such a short distance!

 

100k is good preventative maintenance. i've replaced very old O2 sensors on older soobs with tons of miles and never had a noticeable increase in gas mileage. so i wouldn't replace it unless you have an issue like bad gas mileage.

 

wires can vary from manufacturer to manufacturer and model to model. subaru OEM wires seem very resilient to age and mileage in my experience so i don't see why they won't last until 100k as well. i changed mine at 60,000 on my OBS because i was installing magnecor wires anyway which never need replaced.

[OB99W]

at 186,282 mph i don't know if those electrons are being slowed enough to be significant over such a short distance!

I didn't want to get too deeply into this, which is why I just used the word "slows". However, since you seem somewhat reluctant to believe me, I'll explain a bit more.

 

186,282 mph is the speed of light in a vacuum. It's a common belief that electrons move at that speed in a wire, but it just isn't so. Even under ideal conditions, electron current moves slower than the speed of light; under certain circumstances (due to resistance, capacitance, inductance), dramatically so.

 

Are you familiar with the term "rise time"? When speaking of voltage that is not constant, it's the time it takes for it to go from a certain low value to a certain higher value. Resistance in the secondary ignition circuit, in addition to slightly reducing the available voltage at the plug, causes the rise time of the spark voltage to decrease. The faster a pulse rises, the more its initial harmonic content, and the more likely it is to trigger parasitic oscillations and further harmonic generation; slowing the rise time therefore reduces EMI. At the same time, the slightly lengthened spark duration can be beneficial for ignition.

 

I hope that helped.

[frag]

I didn't want to get too deeply into this, which is why I just used the word "slows". However, since you seem somewhat reluctant to believe me, I'll explain a bit more.

 

186,282 mph is the speed of light in a vacuum. It's a common belief that electrons move at that speed in a wire, but it just isn't so. Even under ideal conditions, electron current moves slower than the speed of light; under certain circumstances (due to resistance, capacitance, inductance), dramatically so.

 

Are you familiar with the term "rise time"? When speaking of voltage that is not constant, it's the time it takes for it to go from a certain low value to a certain higher value. Resistance in the secondary ignition circuit, in addition to slightly reducing the available voltage at the plug, causes the rise time of the spark voltage to decrease. The faster a pulse rises, the more its initial harmonic content, and the more likely it is to trigger parasitic oscillations and further harmonic generation; slowing the rise time therefore reduces EMI. At the same time, the slightly lengthened spark duration can be beneficial for ignition.

 

I hope that helped.

 

Now I know how my students feel when I talk to them about Plato

I think I wont come to close to those wires. I would'nt want any slowing down of my "rise time".

Like the British say, OB99W, just jesting.

Happy new year!

[OB99W]

Now I know how my students feel when I talk to them about Plato

I think I wont come to close to those wires. I would'nt want any slowing down of my "rise time".

Like the British say, OB99W, just jesting.

Happy new year!

Plato was pretty handy with double entendre, too.

Happy new year to you and the other forum members!

[cookie]

Just an observation, recently I changed my front knock sensor on my 99 Forester with 120,000 miles. So far the only difference I notice is that the engines drops revs slower when I stop for a light. I cleared the codes and computer and it seemed to readjust shortly.

I think the car is doing just what it is supposed to be doing, avoiding extra nox production when the throttle is chopped. I am thinking along the lines that my old sensor was too lazy to respond like this. Any other ideas? I have not burned my first tank of gas yet to see how my mileage is affected if at all. This was done because I had code PO420 and I wanted to see if a sensor would fix it.